Help with Genetics for OB!

Esme12, ASN, BSN, RN · Mar 22, 2013

I have to look this up....not my forte. Hopefully someone else will come around...I'm off searching!

Esme12, ASN, BSN, RN · Mar 22, 2013

Ok......In sex-linked inheritance, the gene responsible for the disease is located on the X chromosome. Usually, the abnormal gene is recessive. For these reasons, the resultant disorder is called an X-linked recessive disease.

In a woman with such a defective gene, the effects of the abnormal gene are masked by those of the normal gene on the other X chromosome. Although she does not have the disease herself, she is a carrier, capable of transmitting the defective gene to her children.

In X-linked recessive disease, the Y chromosome lacks the corresponding normal gene to mask the harmful effects of the abnormal gene on the X chromosome. Thus, all male offspring of a woman who is a carrier of an X-linked recessive disease have a 50 percent chance of having the condition. All female offspring have a 50 percent chance of being carriers.

http://staff.washington.edu/sbtrini/Genetic%20Concepts%20and%20Skills/X-Linked%20Recessive%20Inheritance.pdf

I hope this helps :)

NOLAgirl2 · Mar 22, 2013

Esme, that actually helped out A LOT! Thank you! Now it makes sense!

Esme12, ASN, BSN, RN · Mar 22, 2013

Sometimes you can't see the forest for the trees....LOL You're welcome :) anytime!

KelRN215, BSN, RN · Mar 23, 2013

Okay, I dont know if I'm thinking too much into it, but I'm getting VERY confused!
Female is XX, male is XY.
So here is my question: For the X-linked Recessive disorders, why would the MALES be affected and the FEMALES only be carriers if the female is XX. Wouldn't it make sense for the males to be the carrier and the females to be affected by it? Since the females have both XX.
I'm confused by this and its annoying me!

No, let's take an example. A disease like Duchenne Muscular Dystrophy is X-linked recessive.

The mother's X chromosomes are X(M) X(m) where the (m) would be the muscular dystrophy. This mother is a carrier because the (m) gene is recessive and the (M) on her other X is dominant which prevents the disease from being expressed. Her partner is unaffected and his genes are X(M)Y. All males who inherit the X(m) chromosome from the mother will have the disease because they do not have another copy of the X chromosome. Each male child has a 50% chance of having the disease and each female child has a 50% chance of being a carrier.

In X-linked diseases, women are carriers and rarely affected. It would technically be possible for a girl to have X-linked DMD if a carrier mother procreated with an affected father but this would be extremely rare.

nurseprnRN, BSN, RN · Mar 24, 2013

Color blindness is the same way-- it lives on the X chromosome and the vast, vast, vast majority of colorblind people are male. However, it is possible to have a female with colorblindness. My daughter had a classmate, a little girl who was always taking the wrong coat off the rack (same coat, different color) and chose the black crayon all the time because it was the darkest and most definitive. She just didn't seem to pick up the concept of color even though she was a very bright child, began reading well at 3 1/2. So when she was old enough to know her numbers, they gave her the Ishihara numbers test : Take the Ishihara Color Vision Test

Turns out her mother's father was colorblind (there's one x) and her father was colorblind (there's the other one). Her mother was Xx, her father xY. She had a little brother later who turned out to be XY, normal color vision, and another brother who was xY, colorblind like his dad and maternal grandpa.